Melanin and genes

Melanin is produced by cells called alleles, resulting in the great variety of human skin tones.

Melanin controls the amount of ultraviolet (UV) radiation from the sun that penetrates the skin by absorption. While UV radiation can assist in the production of vitamin D, excessive exposure to UV can damage health.

KITLG

ASIP

SLC24A5

SLC45A2

TYR

OCA2

DCT

A number of studies have found genes linked to human skin pigmentation that have alleles with statistically significant frequencies in Asian populations. While not linked to measurements of skin tone variation directly, [28] have been indicated as potential contributors to the evolution of light skin in East Asian populations.

OCA

MC1R

Evolution of skin color

It is theorized that about 1.5 million years ago, the earth endured a megadrought that drove hominids from lush rainforests into arid, open landscapes. This, coupled with the loss of dense body hair, caused early human skin to endure excess UV-B radiation and xeric stress.[47] Evolutionary pressure meant that any gene variations that resulted in lighter skin were less likely to survive under the intense African sun, and human skin remained dark for the next 1.1 million years.

About 70,000–100,000 years ago some modern humans began to migrate away from the tropics to the north where they were exposed to less intense sunlight, possibly in part due to the need for greater use of clothing to protect against the colder climate. Under these conditions there was less photodestruction of folate and so the evolutionary pressure stopping lighter-skinned gene variants from surviving was reduced. In addition, lighter skin is able to generate more vitamin D (cholecalciferol) than darker skin so it would have represented a health benefit in reduced sunlight if there were limited sources of vitamin D.[3] Hence the leading hypothesis for the evolution of human skin color proposes that:

From ~1.2 million years ago to less than 100,000 years ago, the ancestors of all people alive were dark-skinned Africans.

As populations began to migrate, the evolutionary constraint keeping skin dark decreased proportionally to the distance north a population migrated, resulting in a range of skin tones within northern populations.

At some point, northern populations experienced positive selection for lighter skin due to the increased production of vitamin D from sunlight and the genes for darker skin disappeared from these populations.

The genetic mutations leading to light skin, though different among East Asians and Europeans,[5]

There is a long-standing hypothesis that the selection for lighter skin due to higher vitamin D absorption occurred soon after the [50]

One of the most recently proposed drivers of the evolution of skin pigmentation in humans is based on research that shows a superior barrier function in darkly pigmented skin. Most protective functions of the skin, including the permeability barrier and the antimicrobial barrier, reside in the stratum corneum (SC) and the researchers surmise that the SC has undergone the most genetic change since the loss of human body hair. Natural selection would have favored mutations that protect this essential barrier; one such protective adaptation is the pigmentation of interfollicularepidermis, because it improves barrier function as compared to non-pigmented skin. In lush rainforests, however, where UV-B radiation and xeric stress were not in excess, light pigmentation would not have been nearly as detrimental. This explains the side-by-side residence of lightly pigmented and darkly pigmented peoples..[51]

Irregular pigmentation

Uneven pigmentation of some sort affects most people, regardless of ethnic background or skin color. Skin may either appear lighter or darker than normal; there may be blotchy, uneven areas, patches of brown to gray discoloration or freckling. Skin pigmentation disorders occur because the body produces either too much or too little melanin.

Chloasma describes skin discolorations caused by hormones. These hormonal changes are usually the result of pregnancy, birth control pills or estrogen replacement therapy.

Solar lentigo, also known as “liver spots” or “senile freckles” refers to darkened spots on the skin caused by aging and the sun. These spots are quite common in adults with a long history of unprotected sun exposure.

Aside from sun exposure and hormones, hyperpigmentation can be caused by skin damage, such as remnants of blemishes, wounds or rashes.[52] This is especially true for those with darker skin tones.

The most typical cause of darkened areas of skin, brown spots or areas of discoloration is unprotected sun exposure. Once incorrectly referred to as liver spots, these pigment problems are not connected with the liver.

On lighter to medium skin tones, solar lentigenes emerge as small- to medium-sized brown patches of freckling that can grow and accumulate over time on areas of the body that receive the most unprotected sun exposure, such as the back of the hands, forearms, chest, and face. For those with darker skin colors, these discolorations can appear as patches or areas of ashen-gray skin.

Exposure to sun

Melanin in the skin protects the body by absorbing solar radiation. In general, the more melanin there is in the skin the more solar radiation can be absorbed. Excessive solar radiation causes direct and indirect DNA damage to the skin and the body naturally combats and seeks to repair the damage and protect the skin by creating and releasing further melanin into the skin's cells. With the production of the melanin, the skin color darkens, but can also cause sunburn. The tanning process can also be created by artificial UV radiation.

There are two different mechanisms involved. Firstly, the UVA-radiation creates oxidative stress, which in turn oxidizes existing melanin and leads to rapid darkening of the melanin. Secondly, there is an increase in production of melanin (melanogenesis).[53] Melanogenesis leads to delayed tanning and first becomes visible about 72 hours after exposure. The tan that is created by an increased melanogenesis lasts much longer than the one that is caused by oxidation of existing melanin.

A person's natural skin color has an impact on their reaction to exposure to the sun. The tone of human skin can vary from a dark brown to a nearly colorless pigmentation, which may appear reddish due to the blood in the skin. Europeans generally have lighter skin, hair, and eyes than any other group, although this is not always the case. Africans generally have darker skin, hair, and eyes, although this too is not universal. For practical purposes, such as exposure time for [56]

Dark skin with large concentrations of melanin protects against exposure to ultraviolet light and skin cancers; light-skinned people have about a tenfold greater risk of dying from skin cancer, compared with dark-skinned persons, under equal sunlight exposure. Furthermore, UV-A rays from sunlight are believed to interact with folic acid in ways which may damage health.[57]

In a number of traditional societies the sun was avoided as much as possible, especially around noon when the ultraviolet radiation in sunlight is at its most intense. Midday was a time when people stayed in the shade and had the main meal followed by a nap.[58]

While dark skin offers better protection from intense ultraviolet light, it may result in low vitamin D levels and has led to concern that darker skinned people living at relatively high latitude, such as African Americans, may have inadequate vitamin D levels.[67]

In another recent study, the vitamin D levels of traditionally living people in East Africa with skin type 6 was actually measured for the first time; it was found that the mean calcidiol level is 115 nmol/l,[69]

“We expected a much higher status in these people, since they do not spend most of their time behind a PC, under a roof or totally protected by their clothing habits,” Luxwolda said. “However, we were surprised that their status was even much higher than the 80 nmol/l which we had expected, since this seemed most profitable from previous studies with regard to calcium homeostasis and bone fractures.”

Geographic variation

Approximately 10% of the variance in skin color occurs within regions, and ~90% occurs between regions.[72]

Considerable speculation has surrounded the possible adaptive value of other physical features characteristic of groups, such as the constellation of facial features observed in many eastern and northeastern Asians.[75]

Social status and racism

According to classical scholar Frank Snowden, skin color did not determine social status in ancient Egypt, Greece or Rome. Relations between the major power and the subordinate state was viewed as more significant in a person's status than was their skin color.[76]

The preferred skin tone varies by culture and has varied over time. A number of indigenous African groups, such as the Maasai, associated pale skin with being cursed or caused by evil spirits associated with witchcraft. They would abandon their children born with conditions such as albinism and showed a sexual preference for darker skin.[77]

Many cultures have historically favored lighter skin for women. In Europe, before the arsenic to whiten skin, and powders. Other methods included wearing full-length clothes when outdoors, including gloves and parasols.

Colonization and slavery by European countries inspired [84] and elaborated:

We learn as girls that in ways both subtle and obvious, personal and political, our value as females is largely determined by how we look… … For black women, the domination of physical aspects of beauty in women's definition and value render us invisible, partially erased, or obsessed, sometimes for a lifetime, since most of us lack the major talismans of Western beauty. Black women find themselves involved in a lifelong effort to self-define in a culture that provides them no positive reflection.[84]

A Vietnamese motorcyclist wears long gloves to block the sun, despite the tropical heat.

Skin whitening products have remained prominent over time, often due to historical beliefs and perceptions about fair skin. Skin whitening products sales across the world grew from $40 to $43 billion in 2008.[108]

Studies have found that, on average, women of a given ancestry have a lighter skin tone than men of the same ancestry.[109]

Significant exceptions to a preference for lighter skin started to appear in Western culture mid-20th century.[120]

^Sidner, Sara (9 September 2009). “Skin whitener advertisements labeled racist”. CNN.com/asia (CNN). http://www.cnn.com/2009/WORLD/asiapcf/09/09/india.skin/index.html. Retrieved 11 September 2009. “”We always have a complex towards a white skin, towards foreign skin or foreign hair,” Jawed Habib says. Habib should know. He owns a chain of 140 salons located in India and across the world. “We Indian people, we Asian people are more darker, so we want to look more fair.” … A marketing study found sales for skin whitening creams have jumped more than 100 percent in rural India and sales for male grooming products are increasing 20 percent annually.”

http://books.google.com/?id=64xpa-rhHLgC&pg=PA151&dq=sun+tan+lower+class&q=. Retrieved 11 September 2009. “Harris investigated the history of the parasol… everywhere ordinary people were forbidden to protect themselves with such devices “pallid skin became a marker of upper-class status”. At the beginning of the 20th Century, in the United States, lighter-skinned people avoided the sun… Tanned skin was considered lower class.”

External links

N Wade (August 19, 2003), “Why Humans and Their Fur Parted Ways” New York Times (Science Times). Summary of clues to the saga in which humans evolved to lose their hair and had to adjust, including turning from light skin to dark skin, together with an estimation of the time at which humans invented clothing.